The Global Positioning System (GPS) or another global navigation satellite system (GNSS) provides location information to a receiving device anywhere on Earth as long as the device has a substantial line of sight without significant obstruction to three or four satellites of the system. The accuracy of GPS changes over time and is not reliable enough for some applications, or in some areas.
GPS calculations may vary according to the specific GPS receiver or the position and availability of GPS satellites because clustered satellites may cause errors. The accuracy of GPS may depend on the path of the GPS signals as they are affected by objects such as terrain, buildings, or weather. Some GPS signals may reflect from these objects. The GPS receiver may receive both the original, direct signal and the new, reflected signal. The resulting errors introduced in the GPS calculations are multipath errors or multipath interference.